Self-extinguishment of corona discharge in electrical apparatus



Sept. 28, 1965 B. J. EISEMAN, JR ,2

SELF-EXTINGUISHMENT OF CORONA DISCHARGE IN ELECTRICAL APPARATUS Filed Jan. 22, 1962 i T lJf I I I I INVENTOR BERNHARDT J. EISEMAN, JR.

ATTORNEY United States Patent 3 209,063 SELF-EXTINGUISHMENT 0F CORONA DIS- CHARGE IN ELECTRICAL APPARATUS Bernhardt J. Eiseman, Jr., Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington,

Del., a corporation of Delaware Filed Jan. 22, 1962, Ser. No. 167,675 7 Claims. (Cl. 174-17) This invention relates to electrical devices, and more particularly to those employing gaseous dielectrics.

It is known that, in electrical devices such as those in which electrical conductors of different potentials are arranged near each other, various types of discharges between the conductors are possible and all too frequently occur, such as the spark-over or arcing, glow discharge, point discharge, silent electrical discharge, and corona (or crown discharge). Often fluids, both gases and liquids, are used to prevent the arcing or spark-over discharge. Where gases are used as the dielectric, the other types of discharges are quite frequent. The glow discharge, however, which takes place only at greatly reduced pressures, does not ordinarily occur in the general types of electrical apparatus.

The present invention is more particularly directed to a method for reducing the corona discharge which occurs when highly stressed, non-uniform electrical fields exist over restricted areas. In the present invention, the term corona discharge will be used to include the so-called point discharge and silent electrical discharge which in many cases are referred to as corona.

In this type of discharge, ordinarily the current flow is self-limiting although typical examples of a corona discharge occur where voids exist in solid insulation or between conductors which, due to imperfections, do not have a uniform surface. Unlike the glow discharge, the corona discharge occurs at about 0.1 atmosphere pressure and above. While various methods have heretofore been employed in an attempt to control corona discharge, none of them has been completely satisfactory. Unlike arcing or spark-over which causes immediate damage to electrical equipment, the corona discharge gives slow but cumulative effects which in many cases are serious since they cause deterioration of solid, liquid and gaseous insulating materials.

It is an object of the present invention to provide electrical apparatus in which the corona discharge will be self-extinguishing, by employing as the gaseous dielectric, or as an addition to the gaseous dielectric, a material which under the influence of the corona discharge will produce a non-conducting or semi-conducting coating on the conductor at the point or over the area of discharge.

It has now been found that corona discharge in electrical apparatus, in which gases are used as the dielectric material alone or in conjunction with other dielectric ma- 3,209,063 Patented Sept. 28, 1965 operating conditions. The gaseous dielectrics generally employed for this purpose have been found to be ineflieca tive against corona discharge in the apparatus in which they are employed, such as in transformers, shielded high voltage electric cables, capacitors, condensers or similar types of apparatus. In all such devices. conductors under different electric potentials exist relatively close to each other, and in such devices corona can and often does occur particularly where high voltages exist or where such occur even momentarily such as in the starting of electrical motors or where overloads occur in the conductors.

Corona occurs where a field of high electrical stress exists, such as where voids exist in solid insulation on wires or where the normal smooth surface of the conductor is marred by points or burrs such as those which occur on wire due to the imperfections in the wire-drawing process.

Corona often can be detected visually, particularly in total darkness, where it appears as a faint'glow. Other forms of the same phenomena may not be apparent visually. A more convenient and much more sensitive method for detecting corona discharge in any form is by the use of a cathode ray oscilloscope, such as described hereinafter.

As previously stated, the present invention contemplates adding to a gaseous dielectric a gaseous material or a low boiling liquid which under conditions of operation of the apparatus has a substantial vapor pressure and which under the influence of the corona discharge itself forms a resinous material. The compounds used in the present invention are organic compounds which either by themselves or in combination with the gaseous dielectric with which they are used under the influence of an ionizing electrical discharge, form resinous deposits on the condoctor at the point where the corona discharge occurs. Since it has been found that these deposits are either nonconductors or semi-conductors, the extinguishrnent of the corona is believed to be due to the spreading of the stressed electrical field over a larger area and thus diminishing the stress, or by more completely insulating the area where the corona originally takes place.

The corona extinguishing substances of the present invention may also be used as the gaseous dielectric itself if it has the necessary dielectric and other properties. In general, however, they will be used as additives to gaseous dielectrics which do not themselves cause selfextin-guishment of corona. It is of course necessary to have a suflicient amount of the corona extinguishing gas present to produce a coating for the corona-producing area when the corona discharge takes place. The minimum partial pressure of the corona-suppressing substance should be at least about 10 mm. of mercury. Of course no upper limit is set since the corona-suppressing substance may be the dielectric material itself, in which case the total pressure would be that of the corona-suppressing substance.

It is of course understood that there is a corona starting voltage for a given system depending on both the dielectrics and the conductors and the configuration of the conductors. When the starting voltage is exceeded, corona occurs. It is frequently possible to extinguish corona without changing the voltage, such as by increasing the pressure of the dielectric gas or by replacing part of the dielectric gas with a material having better dielectric properties, but these methods give only temporary relief without correcting the factors which tend to produce the corona. Self-extinguishment of corona is used herein to mean that, after the corona starting voltage has been exceeded and corona is occurring, the corona will extinguish itself without any other change being made. This has the advantage of being automatic in nature, whereby the corona is extinguished even without knowledge that the corona discharge is taking place.

In general, the compounds which according to the present invention may be used for self-extinguishment of corona will be those organic compounds which in the presence of an ionizing electrical discharge produce a polymeric coating material, usually resinous in character, which is a non-conductor or a semi-conductor of electricity. A number of gaseous compounds of this type have been described in the published literature. As illustrative of this type of compound may be mentioned: benzene, toluence .perfluoro'benzene, vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, acrylonitrile, butadiene, cyclohexene, ethylene, isobutylene, propylene, cyclohexane, neopentane, -octafluoro-1,4-dithiane, propylene oxide, '1,4-dioxane, and nitromethane.

The above-mentioned compounds which may be used for self-extinguishment of corona will generally be added in relatively small amounts to the usually gaseous dielectric materials, although, as stated above, they can be used along where they exhibit the otherwise required dielectric properties. In addition to the particular dielectric materials disclosed in the specific examples, the following further exemplify the gaseous dielectrics with which the compounds of the present invention may be employed: 2-chloroheptafluoropropane, decafluorobutane, chloropenta'fluoroethane, octafluoropropane, hexafluoroethane, 1,Z-dichlorotetrafluoroethane, 1,1,2-t-richlorotrifiuoroethane, perfluoro-(Z-n-butyltetrahydrofuran), perfluorotrimethylamine, chlorotrifluoromethane, carbon tetrachloride, nitrogen, helium, hydrogen, carbon dioxide. It is of course understood that the corona-extinguishing compounds of the present application may be used with any gaseous dielectric since quite obviously under the conditions employed in their use as dielectrics there will normally be no reaction between the corona extinguishing compounds and the dielectric materials.

The following examples are based on tests of the corona extinguishing compounds which have been carried out in an apparatus more particularly illustrated in the attached drawing which forms a part of the present application. In this drawing, FIGURE 1 shows the test apparatus, while FIGURE 2 is a wiring diagram for a cathode ray oscilloscope circuit used in detecting corona discharge even where it was not apparent visually.

The apparatus of FIGURE 1 of the drawing comprises a common three-necked, round-bottomed flask 1, which can conveniently be of 500cc. capacity. Rubber stoppers (polychloroprene) 2, 3 and 4 were used. Through stopper 2 there is sealed a copper wire 5 to which is attached a small alligator clamp 6 in which a steel needle 7 is held. Through stopper 3 is sealed a A" outside diameter copper tube 8, the end 9 of which is crimped closed and soldered. The end of needle 7 is adjusted to about A" from the nearest point on the tube 8. Through the center stopper 4 an inlet copper tube 10 is provided to permit the removal and introduction of various gaseous materials to the system. The leads of the secondary winding of a 10,000 volt transformer 11, see FIGURE 2 (secondary rated at 23 milliamperes), are attached through a circuit as illustrated in FIGURE 2 of the drawing to electrodes 5 and '8. The primary windings of the transformer are attached to a source of electric power through a variable transformer d2 so that there can be supplied to the primary windings a voltage ranging from 0 to 130 volts of alternating current.

Observations were sometimes made visually by connecting the 10,000-volt transformer, 11, directly to the electrodes 5 and 8, leaving out the remainder of the circuit of FIGURE 2.

The tests were carried out at approximately room temperature (about 77 F.). If a single substance was being tested, the flask was evacuated to approximately 0.05 mm. Hg. pressure and the material to be tested was then introduced into the flask. Where the material to be tested was a gas at room temperature and atmospheric pressure, suflicient of this gas was introduced to bring the pressure to the desired level as given in the specific examples. If the material was a liquid, the vapor of the liquid was introduced into the flask to provide the vapor pressure specified in the examples. When mixtures of gases were used, or mixtures of gases with relatively volatile liquids, the gas of lower molecular weight Was introduced first, then the second gas of higher molecular weight was introduced. The partial pressures of each material of course determined the mol ratio of the materials that were used in the flask. Usually where mixtures were employed, a small area of the side of the flask was heated to cause convection in the cell to produce more rapid mixing of the gases. This was repeated several times to provide thorough mixing. Where vapors of liquids are used, the liquid itself may be introduced into the flask to provide the desired vapor pressure. For best results, the rubber stoppers in the flask were changed frequently to prevent absorbed materials from contaminating materials in subsequent tests.

While the corona in many cases could be observed visually when the test was run in a'darkened room, the use of a cathode ray oscilloscope gave much more accurate determination of the presence or absence of corona discharge.

In the following examples the voltage applied as read on the voltmeter 13 was generally increased until corona was observed either visually or by the oscilloscope. In the examples the voltage given is that actually used on the secondary winding of the transformer calculated from the voltmeter reading of the voltage of the primary winding as shown on the drawing, using a factor of 86.8. If the corona extinguished itself, the eifect of increased voltage was determined. It is to be understood that there is a limiting voltage that can be applied to any system, above which arcing takes place. The voltage which can be used is determined by the materials in the system and their pressures. If the voltage is maintained near the sparkover or arcing voltage, permanent suppression of corona will usually not be obtainable, so that the tests are run at voltages below that which results in arcing.

FIGURE 2 of the drawing illustrates the external circuit employed with the apparatus of FIGURE 1 where a cathode ray oscilloscope 16 is used. Since this oscilloscope is a standard commodity of commerce, the internal circuits are not shown. The circuit shown inside the dotted-line rectangle produces an elliptical Lissajous figure as at 15 on the oscilloscope 16, with the major axis substantially horizontal. The vertical dimension of the Lissajous figure was adjusted by means of a K ohm resistor 17. When corona occurred, it appeared on the oscilloscope as vertical lines superposed on the Lissajous figure 15. The electrodes from the test cell or flask 1 in FIGURE 1, which in FIGURE 2 is indicated diagrammatically at 1, were arranged so that the needle side or copper wire 5 of the test cell circuit was connected directly to the oscilloscope 16, and a 30 millihenry high frequency choke 21 was inserted between the needle circut and the circuit which generated the Lissajous figure 15. A protector 22 of a special type consisting of a piece of silicone-treated paper between a plate and a spring electrode was provided to protect the oscilloscope in case of flashover between the needle 7 and tube 8 in FIGURE 1.

The following specific examples are given to more fully illustrate the invention. In the first five examples, the

a majority of the tests were carried out at approximately 92 to 94 mm. Hg pressure, since at this pressure the light to be visually observed is more readily and conveniently obtained than at higher pressures. Too low a pressure is of course avoided, such as would give the glow phenomena previously referred to.

Example 1 The test cell described above was evacuated to less than 0.05 mm. Hg pressure. Then, octafluoro-lA dithiane was admitted as the saturated vapor at room temperature (92 mm. Hg pressure). Sixty cycle alternating current was applied and the voltage was gradually raised. The results obtained by visual observation were as follows:

Time elapsed, Voltage Observation minutes 2 2, 640 Small blue corona, at

needle tip, twinkled a few times and disappeared.

6 3, 170 Corona restored which twinkled, giving intermittent corona.

20 3, 170 Corona extinguished. 27 3, 170 Corona still completely extinguished.

Under 50 magnification, the tip of the needle was found to be covered with a transparent cap of resinlike material. There was no deposit on the copper tube elec trode.

- Example 2 In the same manner as Example 1, benzene at 88 mm. Hg pressure was admitted to the test cell. The following observations were made:

Time elapsed, Voltage Observations minutes 1 1, 350 Twinkled once. 3 1, 530 Twinkled a few times only. 5 1, 640 Twinkled off and on, with off time increasing. 17 1, 640 Corona extinguished. 27 1, 640 Corona remained extinguished.

Time elapsed, Voltage Observations minutes 1 1, 310 Faint twinkling; stopped. 4 1, 910 Do. 5 2, 100 Corona, but weakening. 12 2, 100 Corona extinguished. 14 2, 370 C(tnona reestablished,

w 22 2, 370 Corona extinguished completely for a number of minutes. i

The needle tip had a deposit of dark, resinous material on it. Thus, mixtures of benzene and sulfur hexafluoride extinguish corona.

In the same manner, a mixture containing 32 mol percent of benzene and 68 mol percent of perfluorocyclobutane at a total pressure of 114 mm. was tested. The following was observed:

Time elapsed, Voltage Observations minutes Tiny twinkle. Faint corona, some twin- Corona extinguished.

Corona reestablished, goes ofi and on.

Corona extinguished.

Corona remains extinguished.

Slight twinkle.

Corona extinguished completely for the last 5 minutes.

pp mm 0000 0000 00:: BIO! BIC! Q0 co co co This example demonstrates that benzene, in mixtures with substances which do not extinguish corona, also extinguishes corona.

Example 4 It was found that each of the mixtures shown in the following table eifectively extinguished corona under approximately the same conditions as described in Example 3.

Total Proportion Coronarsuppressing Dielectric Gas B pressure, A/B by mol Substance A with which tested mm. Hg. percent Vinyl fluoride Sulfur hexafluoride 94 32/68 Acrylonitrile ('in 94 32/68 :Bntadiene 94 32/68 Cyclohexane 94 32/68 Cyclohexene. 94 32/68 Eth ene. 94 32/68 Isobutyleue 94 32/68 Neopentane 94 32/68 Nitromethane. 94 32/68 Propylene 94 32/68 Propylene oxide. 94 32/68 Vinylidene fluoride 94 32/68 Octafluoro-1,4 188 50/50 dithiane.

Do 94 32/68 Carbon disulfide do .l- 94 32/68 Do Octafluorocyclo- 94 32/68 butane. 1,4 dioxane Sulfur hexafluoride 94 34/66 Tetrafluoroethylene do 94 32/68 Toluene do 94 28/72 Benzene do 1 180 17/83 Octafluoro-1,4- Octafiuoroeyclo- 94 32/68 dithiane. utane.

Do do 188 50/50 Perfiuoro(2-n-butyl- Sulfur hexafluoride 180 16/84 tetrahydrofuran) Example 5 The following examples were carried out as in Example 3, except that the steel sewing machine needle (7 in FIGURE 1) was replaced with a piece of copper wire (B & S #22) filed to a sharp point. In each case corona was extinguished under approximately the same conditions as given in Example 3.

Total Proportion Corona-suppressing Dielectric Gas B pressure, A/B by mol Substance A with which tested mm. Hg. percent Ben en 88 /0 D Sulfur hexafluoride. 94 32/68 Octafluorocyclo- 94 32/68 butane 94 100/0 Sulfur hexafluoride 94 32/68 Octatluorocyclo- 94 32/68 utane. Octafiuoro-1,4- 94 100/0 dithiane. Propane Sulfur hexafluoride 94 32/68 Propylene oxide do 94 32/68 Do Octafiuorocyclo- 94 I 32/68 butane.

Thus, the nature of the electrode ,(i.e., steel or copper) has no eifect on the extinguishment of corona.

Examples 6 to 8, inclusive, were carried out at ordinary room temperature and atmospheric pressure.

7 Example 6 Using the test apparatus described above, except that the needle was placed about one-half inch from the copper tube, a test was carried out with sulfur hexafluoride at one atmosphere pressure absolute. At a voltage of about 6600, light appeared and remained steady by visual observation at voltages of 6600 to 6700 for 65 minutes.

Octafluor-o-l,4-dithiane was then added to form a small pool in the bottom of the flask. After resealing, the system was partially evacuated until rapid evaporation of the dithiane occurred (liquid still remained); then sufficient sulfur hexafluoride was added to bring the total pressure to one atmosphere absolute. Mixing was carried out by heating a small area of the flask, causing convection in the vapors (vapor pressure of octafluoro- 1,4-dithiane equals 90 mm. Hg). Voltage Was then ap plied to the electrodes with the following results:

Time elapsed, Voltage Observations minutes 6, 700 No visible light. 1 8, 000 Faint light. 6 8, 000 Light flickering. 9 8, 000 Light extinguished.- 32 8, 000 Light remained extinguished; power off. 7,130 Power on; no light. 86 s, 090 No light. 89 10, 260 Light flickered and disappeared. 92 10, 260 No light. 102 10, 260 Do.

The needle electrode was later found to have a white, slightly rough-appearing resinous cap on its tip.

Example 7 i In the same manner as the previous example, a mixture of benzene and sulfur hexafluoride (benzene partial pressure equals 100 mm., total pressure equals 1 atmos.) was tested with the following results:

Time elapsed, Voltage Observations minutes 1 6, 000 Weak, flickering light. 4 6, 520 0. 6 6, 520 Llgjlrt off and on, mostly o 8 6, 520 Light off. 30 6, 520 Light remained off.

On examination, the needle electrode was found to have a small, lumpy, resinous coating at the tip.

Example 8 Example 7 was repeated, substituting ostafluorocyclobutane for sulfur hexafluoride. The following observations were made:

NO BENZENE PRESENT Time elapsed, Voltage Observations minutes 8, 090 Weak light. 31 8, 090 Light remained.

BENZENE PRESENT (100 MM. Hg PARTIAL PRESSURE, TOTAL PRESSURE 1 ATMOS.)

4 7, 830 Weak light. 12 7, 830 Light extingu1shed. 31 7, 830 Light remained extinguished.

The needle electrode was later found to have a trans parent, resinous deposit covering the tip with a dark spot on one side.

(where the corona occurred) were shown to be semiconductors or non-conductors by the following tests:

One lead of a direct current resistance measuring unit was connected to an unused steel needle and the other to a pool of mercury via an immersed electrode. The ohmmeter indicated infinite resistance as the needle was lowered toward the mercury pool until contact was made when zero resistance was indicated, i.e., a short circuit. Needles used in the corona supression tests were carried through the same test. As the tip of the needles came in contact with the mercury, a zero resistance was not obtained. Instead, either infinite resistance or a rather high resistance, but not infinite, was obtained. The intermediate resistance indicated a semi-conductor.

The semi-conductive nature of the deposits was confirmed by putting a 10,000-ohm resistor in series with the needle, connecting an oscilloscope across the resistor, and applying a fraction of a volt to 6 volts 60-cycle alternating current to the circuit. The amplitude of the cycles was increased or decreased with needle immersion depth, again indicating the semi-conductive nature of the deposit.

Tests of the various deposits which were found to be present, when tested by the above methods, gave the following results:

Source of deposit Test Results A.O.-scope D.O.-ohmmeter.

Non-conductor. Infinite resistancenon-conductor. Semi-conductor.

. Cyclohexene-SF, Cyelohexane-SF Cyclohexane-SF Optafiuoro-1,4-dithiane Benzene-SF Perfluoro-(iZ-n-butyltetrahydrofuran) Non-conductor.

It was found in each case that the deposit present on the needle after the corona suppressing tests had been run was either a semi-conductor or non-conductor.

The corona-suppressing gases of the present invention may be used in all types of electrical apparatus where corona discharge takes place, such as in ordinary electrical transformers, vapor-cooled transformers such as disclosed in U.S. Patents 2,777,009 and 2,886,625, communication cables such as described in U.S. Patent 2,221,670, power cables where gaseous dielectrics are employed, capacitors, electronic equipment, or wherever it is desired to prevent permanent damage to the installation by corona discharge.

I claim:

1. A gas-tight electrical apparatus containing at least one electric conductor from which corona-type discharge may occur, a gaseous dielectric surrounding said conductor; said gaseous dielectric comprising as a corona selfsuppressing agent octafiuoro-1,4-dithiane, the partial pressure of the corona-suppressing agent being at least 30 mm. of mercury.

2. A gas-tight electrical apparatus containing at least one electric conductor from which corona-type discharge may occur, a gaseous dielectric surrounding said conductor; said gaseous dielectric comprising as a corona selfsuppressing agent vinylidene fluoride, the partial pressure of the corona-suppressing agent being at least 30 mm. of mercury.

3. A gas-tight electrical apparatus containing at least one electric conductor from which corona-type discharge may occur, a gaseous dielectric surrounding said conductor; said gaseous dielectric comprising as a corona selfsuppressing agent benzene, the partial pressure of the corona-suppressing agent being at least 30 mm. of

mercury.

4. A gas-tight electrical apparatus containing at least one electric conductor from which corona type discharge may occur, a gaseous dielectric surrounding said conductor; said gaseous dielectric comprising as a corona selfsuppressing agent neopentane, the partial pressure of the corona-suppressing agent being at least 30 mm. of mercury.

5. A gas-tight electrical apparatus containing at least one electric conductor from which corona-type discharge may occur, a gaseous dielectric surrounding said conductor; said gaseous dielectric comprising as a corona selfsuppressing agent propylene oxide, the partial pressure of the corona-suppressing agent being at least 30 mm. of mercury.

6. A gas-tight electrical apparatus containing at least one electric conductor from which corona-type discharge may occur, a gaseous dielectric surrounding said conductor; said gaseous dielectric comprising as a corona selfsuppressing agent nitromethane, the partial pressure of the corona-suppressing agent being at least 30 mm. of mercury.

7. A gas-tight electrical apparatus containing at least one electrical conductor from which corona-type discharge may occur, a gaseous dielectric surrounding said conductor; said gaseous dielectric comprising as a corona selfsuppressing agent an organic compound having a partial References Cited by the Examiner UNITED STATES PATENTS 2,949,424 8/60 Mandelcorn 17417 X 3,001,050 9/61 Collier 174-17 FOREIGN PATENTS 525,244 8/ 40* Great Britain. 539,749 9/41 Great Britain.

OTHER REFERENCES Charlton et al.: Dielectric strength of insulating fluids, General Electric Review, September 1937, pages 438442.

LARAMIE E. ASKIN, Primary Examiner.

25 JOHN P. WILDMAN, Examiner. 

7. A GAS-TIGHT ELECTRICAL APPARATUS CONTAINING AT LEAST ONE ELECTRICAL CONDUCTOR FROM WHICH CORONA-TYPE DISCHARGE MAY OCCUR, A GASEOUS DIELECTRIC SURROUNDING SAID CONDUCTOR; SAID GASEOUS DIELECTRIC COMPRISING AS A CORONA SELFSUPPRESSING AGENT AN ORGANIC COMPOUND HAVING A PARTIAL PRESSURE OF AT LEAST 30 MM. OF MERCURY AND SELECTED FROM THE GROUP CONSISTING OF OCATFLUORO 1,4-DITHIANE, VINYLIDENE FLUORIDE, BENZENE, NEOPENTANE, PROPYLENE OXIDE, AND NITROMETHANE, SAID ORGANIC COMPOUND PRODUCING A RESINOUSTYPE POLYMERIC COATING MATERIAL OF THE CLASS CONSISTING OF NON-CONDUCTORS AND SEMI-CONDUCTORS OF ELECTRICTY OVER SAID ELECTRICAL CONDUCTOR WHEN IN THE PRESNECE OF IONIZING ELECTRICAL DISCHARGE, WHEREBY THE IONIZING ELECTRICAL DISCHARGE IS EFFECTIVELY EXTINGUISHED. 